There have been a bazillion posts like this one:
My fermentation is really (fill in the blank). My airlock is (fill in the blank). Am I going to be okay?
This is not that post.
I just want to know what causes all the movement during an active ferment? I’m thinking of the single cell. Are the yeast cells absorbing and producing stuff and that collectively creates that swirling? If the cells had little mouths or flippers, I could get it. Should I be thinking a billion little sponges expanding and contracting? I need some kind of analogy - or maybe I don’t - to understand. I get that the yeast is “consuming” the sugars, but I don’t get how that causes the motion. I think it’s all this energy that often causes temps to rise, but since I can’t quite conceptualize the cause of the motion, I can’t quite conceptualize the cause of the heat either.
Hope this doesn’t come off as sounding crazy. I’m just curious as the active ferment is my third favorite part of brewing. Smelling the boiling wort is my second favorite and, of course, drinking the finished product is what it’s all about.
:cheers:
Put simply, the swirling of the wort is caused by convection, which occurs as a result of the heat produced by the yeast. As the wort is heated, it rises, and as it cools off (through contact with the sides of the fermenter), it falls, producing the swirling motion you’re seeing.
Thanks Marty. Now I understand the swirling. So what is causing the temp to rise? and would it be correct to say that there is a great deal of temp fluctuation going on with all that movement - a kind of momentum of change?
I believe that CO2 evolution is a major driver of wort movement – microscopic bubbles of CO2 adhere to yeast cells and clumps of trub, causing them to rise to the surface. Thermal convection certainly can play a role, but from my reading it is more an issue in cylindroconical fermenters with jackets where the beer gets cooled in certain zones and then falls.
[quote=“jtb”]Thanks Marty. Now I understand the swirling. So what is causing the temp to rise? and would it be correct to say that there is a great deal of temp fluctuation going on with all that movement - a kind of momentum of change?
I sucked at Physics so I appreciate your help.[/quote]
That’s a pretty good way of putting it.
The rise in temperature is caused by the yeast converting sugar into alcohol. It’s sort of similar to the body heat caused by a human being (or any warm blooded animal, really) as it’s cells convert sugar into energy. And yeah, there’s a considerable amount of heat being generated by the yeast in a batch of beer, enough that the temperature of actively fermenting wort is generally several degrees above ambient. That said, a difference of a couple of degrees in a volume of fluid the size of a batch of beer is generally more than enough to cause the convection you’re seeing in your wort.
Edit: bucketbiochemist brings up a good point. I’m sure, as he says, that CO2 formation is a factor as well, though I’m still sure that convection plays a significant part. Look at the example of a kettle of water heating on a stove. There is generally only a few degrees difference between the water temp at the top versus the bottom, yet you can visibly see the convection in it as well. The water is heated at the bottom and rises, then is cooled at the surface and along the sides of the kettle and falls.
This is one of those questions that would require a model of the system be constructed and run on a substantial computer system to answer with certainty. Fluid dynamic can get amazingly complex. A few years ago a group published a study on why a shower curtain pulls in towards the shower spray, and found it had to do with the creation of angled vortexes that no one had ever expected were there.
I would guess that both Marty and Bucketbiochemist are right.
The heat is caused by respiration of the yeast. Breaking down the sugar and converting it to alcohol and CO2 (and energy for the yeast cell’s own use) releases waste heat. There is a small gradient in the temperature of the wort between the edges, where heat is able to flow out into the environment, and the center where it has no where to go. This creates slightly less dense wort in the center which would tend to push up and displace the wort at the edges that would sink down, creating a donut-shaped flow cell.
But anyone who has watched beer fermenting in a carboy will know that it doesn’t look like that. You see stuff rising from the bottom to the top everywhere, not just in the center. That is likely due to bits of trub sticking to CO2 bubbles that rise up to the surface. And the forces generated by the buoyant gas is more than enough to overcome any currents generated by the mixing due to temperature differences.
At least, that’s my guess until someone does a real study on the issue.
This is one of those questions that would require a model of the system be constructed and run on a substantial computer system to answer with certainty. Fluid dynamic can get amazingly complex. A few years ago a group published a study on why a shower curtain pulls in towards the shower spray, and found it had to do with the creation of angled vortexes that no one had ever expected were there.
I would guess that both Marty and Bucketbiochemist are right.
The heat is caused by respiration of the yeast. Breaking down the sugar and converting it to alcohol and CO2 (and energy for the yeast cell’s own use) releases waste heat. There is a small gradient in the temperature of the wort between the edges, where heat is able to flow out into the environment, and the center where it has no where to go. This creates slightly less dense wort in the center which would tend to push up and displace the wort at the edges that would sink down, creating a donut-shaped flow cell.
But anyone who has watched beer fermenting in a carboy will know that it doesn’t look like that. You see stuff rising from the bottom to the top everywhere, not just in the center. That is likely due to bits of trub sticking to CO2 bubbles that rise up to the surface. And the forces generated by the buoyant gas is more than enough to overcome any currents generated by the mixing due to temperature differences.
At least, that’s my guess until someone does a real study on the issue.
I believe it’s both heat and CO2. Fermentation generates both. And, just because you can’t see actual CO2 bubbles or other gases produced in the fermentation, doesn’t mean it isn’t there on the molecular cell level. So you get the rise and fall. Pretty awesome to watch.
A$$ kickers indeed! I had a fresh smack pack of 2308 Munich Lager yeast and tried to maximize my volume in a 2000 ml Erlenmeyer flask and overnight the yeast decided they didn’t need no stinkin’ foam stopper!
